The optimal medical and surgical management for children with aortic coarctation remains controversial: early surgery prevents the development of sustained hypertension but may leave a residual gradient; late surgery enjoys excellent technical success but may leave the child with life-long arterial hypertension; and a two stage surgical approach suffers because reoperation for recoarctation is hazardous procedure. Similarly, the mechanism of the hypertension that precedes and follows coarctation relief is currently unknown. While most hypothesis suggest that there are abnormalities in blood flow distribution (i.e. renal, adrenal) in aortic coarctation both at rest and during physiologic stress (i.e. exercise), there are no studies which have studied the effects of coarctation relief on flow distribution in the newborn or older animal. The advent of a successful balloon dilation catheter for the treatment of peripheral atherosclerotic lesions seems to offer a solution to both of these difficulties. If balloon dilation of aortic coarctations in newborns were successful, one could initially dilate the lesion transcutaneously (thus eliminating the gradient) and still allow a clean and safe operative field for subsequent difinitive repair. One could also create an experimental coarctation, allow the animal to grow, and determine the flow distribution in that animal at rest and during exercise both before and after gradient relief. Correction would require neither a thoracotomy nor a prolonged convalescence. Therefore, one could use the microsphere technique to measure blood flow distribution during exercise after gradient relief, and still retrieve the microspheres within 72 hours of the first injection. There are four aims of this proposal: 1) To create a reproducible, dilatable form of aortic coarctation in the newborn lamb, and to determine its "natural" history using hemodyamic, angiographic, and pathologic techniques; 2) to determine the safety and efficacy of balloon dilation both acutely and chronically; 3) to determine the mechanism by which balloon dilation provides gradient relief, and the mechanism by which the lesion of dilation heals itself, and 4) to determine the impact of gradient relief on patterns of flow distribution in the newborn lamb with aortic coarctation.